1. Field of the invention
The present invention relates to a pipe of socket and spigot type adapted for pipe jacking having a spigot whose internal and external diameters are larger than those of the pipe body.
2. Prior art
So-called pipe jacking is a method in which, for burying a water main, the rear end of a metal straight pipe is almost horizontally inserted in the earth at a launch pit with pressure applied by a hydraulic jack or the like without excavating the ground, then the spigot of the next straight pipe is joined in the socket at the rear end of the preceding pipe, These pipes are also inserted in the earth with pressure applied by the hydraulic jack. These steps are repeated to form a required pipe line.
In this method, however, a drawback exists in that because of the enlarged spigot of the pipes, considerable resistance is unavoidable when carrying out forward movement under the ground. To overcome such a drawback, a countermeasure is employed, wherein a flange is circumferentially provided on the spigot of each pipe so as to a receive a driving force through a flange portion on the end face of a preceding pipe, and furthermore the outer periphery of the pipe is coated with cement mortar in such a manner that the outer periphery of the pipe between the flange on the spigot and the socket may be almost equivalent to the largest external diameter of the socket. Other countermeasures such as welding of ribs between the flange on the spigot and pipe body, incorporation of reinforcing rods in the coating cement mortar were also adopted in view of reinforcement.
These conventional and customary pipes for pipe jacking still have various disadvantages caused by the instability of the cement-mortared surface, intricate manufacturing or assembling processes, etc., and therefore to meet such disadvantages the applicant already proposed an improved pipe adapted for pipe jacking as is disclosed in Japanese Utility Model Registration Application No. 62-13065 as illustrated in FIG. 6. This preceding improved pipe adapted for pipe jacking is featured in that the spigot 4a is provided with an annular flange 12 and reinforced with a rib 13, and an outer pipe 5a having a smooth external surface is fixedly placed at the outer edge between the spigot 4a and that of the socket in longitudinal direction. Furthermore an annular gap formed between the outer pipe and the pipe body is filled with a filler 6a such as sand or foaming mortar. As a result of such features, several advantages are achieved such that frictional resistance between the outer surface of the pipe and earth is reduced enabling the pipe to be easily moved forward by pipe jacking, with loss of the cement mortar being prevented, and manufacturing as well as pipe carrying being easily achieved.
It is certain that the preceding improved pipe adapted for pipe jacking overcomes the disadvantages of the known and customary pipe coated with cement mortar, but in view of recent remarkable progress in the field of pipe line laying method which has brought several conditions largely different from the conventional pipe jacking, it is now necessary to review requirements of the pipe. For example, a pipe replacement method without open cut has been put into practical use. According to this new pipe replacement method, when an old pipe line becomes obsolete, the old pipe line is replaced with a new one while crushingly removing the former, being different from the conventional pipe jacking which is employed only when new pipe line is constructed without excavation.
FIGS. 7 A and 7 B show a specific example of such a new pipe replacement method. In FIG. 7 A, a mole 15 moves forward into an old pipe line 14 while crushing it, then temporary pipes 16 to be connected to the rear part of the mole and moved following the mole are placed one by one in the area where old pipes have been crushed. Then, as illustrated in FIG. 7 B, after completing replacement of the old pipe line with such temporary pipes over the full length, new pipes 1b to be jacked are inserted one by one from a launch pit 17 and the rear part of the pipes lb is pushed forward by a hydraulic jack 18, whereby the first temporary pipe 16 is pushed out to a reception pit 19 to be removed up to the outside. These steps are repeated to complete replacement of the temporary pipes with a new pipe line.
In the pipe jacking as described above, since new pipe to be jacked is moved forward in the horizontal cavity already provided under the ground by the old pipe line while pushing temporary pipes and/or preceding pipe to be jacked, the required jacking force can be much smaller than that in the conventional pipe jacking. Accordingly, it is not always reasonable to provide a circumferential flange on the spigot of such new pipe for preparation of external arrangement and, moreover, in the assembling process, it will be difficult to carry out fitting such outer pipe serving as an outer casing over the pipe body if a flange or rib is provided beforehand. In particular, it will actually be very troublesome work to fill closed space partitioned by the flange on the spigot, socket and pipe with a filler while exhausting the air therefrom.
Even in the conventional construction using pipe jacking, the pipe jacking distance may be sometimes short when carrying out such a construction as in just crossing road or a railroad, in which the required jacking force can be very small. In such a case, it is also not always reasonable to provide a circumferential flange on the spigot for the preparation of external arrangement.